The invention relates to a process for producing methyl methacrylate (MMA).
MMA is a widely-produced industrial chemical that readily polymerizes. Typical end-use applications include: acrylic plastic sheeting; molding resins; polyvinyl chloride modifiers; processing aids; acrylic lacquers; floor polishes; sealants; auto transmission fluids; crankcase oil modifiers; automotive coatings; ion exchange resins; cement modifiers; water treatment polymers; electronic adhesives; metal coatings; and acrylic fibers. Methacrylate esters are especially prized in these applications and others because of the hardness they impart to the products in which they are used. The most popular industrial process for making MMA is the acetone cyanohydrin (“ACH”) process. Methacrylate ester plants produce extremely large volumes of product; thus, any improvement in process yield, however slight, can have a significant positive economic impact.
In a conventional ACH process for the production of MMA, ACH is hydrolyzed in the presence of sulfuric acid to produce α-hydroxyisobutyramide (“HIBAM”) and α-sulfatoisobutyramide (“SIBAM”). Next, the HIBAM and SIBAM are cracked to form methacrylamide (MAM) and by-products. The MAM is then esterified with methanol to produce the desired MMA product. The esterification product stream is a mixed product that is subjected to separation and purification steps to isolate the MMA product from the other compounds. Typically, a purified MMA product stream is produced, along with a purification residue comprising other compounds.
U.S. Pat. No. 7,253,307 prescribes the use of polymerization inhibitors to minimize polymer formation in a methyl methacrylate/methacrylic acid production process. The exhaustive list of inhibitors includes phenothiazine and its derivatives, hydroquinone and its derivatives, alkoxy-phenols, nitrosophenol and its salts, copper salts, and radical traps such as 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy. The process relies on free-radical inhibitors present in the feed streams to the esterification reactor to prevent polymer formation in the reactor. These inhibitors are designed primarily to limit the concentration of free radicals and free radical polymerization, which leads to equipment fouling. However, despite the use of polymerization inhibitors, commercial MMA plants continue to face significant fouling issues.
In view of the shortcomings of the prior art, it would be desirable to have an improved process that would reduce unwanted polymerization in the production of MMA.